Semiconductor substrates in conventional chip stacks are typically thinned using a mechanical backgrinding process. Backgrinding imparts a high level of mechanical stress to the devices and can result in substantial thickness variation. Therefore, other processes for separating substrates are desirable.
One approach to substrate thinning is described in U.S. Pat. No. 6,316,333 (hereinafter, “Bruel”). Bruel describes implanting ions through gate structures to form a cleaving plane in a substrate and removing a portion of the substrate by cleaving along the cleave plane. Bruel acknowledges that ion implantation causes damage to device structures, e.g, channel regions, that can render the devices inoperable. Bruel describes building structures on the exposed surface of a substrate to selectively block ion implantation, thereby reducing damage to structures that are disposed directly beneath the blocking structures.
However, there are several limitations to Bruel's proposal. The structures described by Bruel are relatively large, e.g. gate lengths of 0.5 microns. Current devices use structures that are much smaller, e.g. gate lengths of 30 nanometers or less, which are more than an order of magnitude smaller than the gate length described by Bruel. In order to accumulate sufficient hydrogen ions to perform a cleaving operation, ions must be implanted through a substantial portion of a device surface. Moreover, modern devices are increasingly complex and include higher quantities of sensitive structures. Some of these structures, such as vertical transistors, have vertical components that are longer than horizontal components, which presents a greater opportunity for damage from a vertically oriented ion that passes through the structure.
In addition, larger structures are generally more robust to ion damage than smaller structures. A smaller structure will have fewer atoms and be more sensitive to the disruption of an atom within the structure. For example, a barrier layer that has a feature size of 10 nm may have a thickness in the tens of atoms, so that disruption of a single atom may have a significant effect on the barrier property.